1. Technical Field
The present disclosure relates to nanomaterial technology and, more particularly, to a membrane or substrate having carbon nanotubes introduced and/or immobilized therein and method for introducing and/or immobilizing carbon nanotubes in membranes or substrates.
2. Background Art
In general, membranes are permeable structures that facilitate the separation of solutes based on size and/or physical and chemical properties. Typical synthetic membranes may be fabricated from a variety of materials, such as, for example, metallic, ceramic or polymeric materials. Over the past few decades, membrane technology has generally made strides by developing materials that allow greater flux and selectivity (Ref. 1). In general, flux is associated with the high permeability of the solutes, and the selectivity is associated with the preferential elimination of interfering species. Typical membranes represent a compromise between these two factors (e.g., membranes with high selectivity tend to have lower permeability and vice versa).
Assessments of permeability and selectivity have generally shown asymptotic limitations on the separation capability of substantially pure polymeric membranes (Refs. 2, 3). Consequently, the development of novel membrane systems is of great importance. One approach has been the development of mixed matrix membranes (“MMMs”), which typically combine polymeric materials with inorganic fillers such as, for example, zeolites (Refs. 4, 5). In general, these MMMs have exhibited greater permeation rates and selectivity in gas separation (Refs. 6-8), higher flux in pervaporation (Refs. 9, 10), enzyme concentration (Ref. 11) and protein separation (Ref. 12). Typical fabrication processes for MMMs involve adding the filler material to the polymer solution followed by film casting or spinning (Refs. 4, 5, 13). In general, these processes are complex, time consuming, require strong interactions between the polymer and the inorganic filler, and they coat the particle with the polymer (Refs. 5, 14-16).
In general, carbon nanotubes (“CNTs”) typically are graphene sheets rolled into tubes as single-walled nanotube (SWNT) or multiple-walled nanotube (MWNT) structures. CNTs can be utilized for membrane systems. There has been interest in CNTs because of their desired thermal, electrical and structural properties (Ref. 17). Some studies have investigated their interaction with organic molecules, and have demonstrated sorbent properties that in some cases are superior to conventional materials such as, for example, C18 and C8 (Refs. 18-20). It has also been reported that self-assembled nanotubes are highly effective as high resolution gas chromatography stationary phases (Refs. 21-24). CNTs have been deposited on ceramic matrices via chemical vapor deposition to form membranes that exhibit high permeation rates (Ref. 25), and aligned MWNTs have facilitated the flow of small organic molecules (Ref. 6). In addition, theoretical studies have suggested that permeabilities of certain liquids and gases through carbon nanotubes far exceed what is expected from classical diffusion models (Refs. 25-28). This enhancement has been attributed to the generally smooth CNT surface, substantially frictionless rapid transport, and molecular ordering (Ref. 27).
In addition to being generally effective transporters (Refs. 25-28), CNTs are typically also effective sorbents, particularly for organics (Refs. 22-24). Together these two properties may increase the selective partitioning and permeation of the solute of interest. In typical membrane-based liquid extractions, when the two phases contact at the pores, the interactions can take place via rapid solute exchange on the CNTs, thus increasing the effective rate of mass transfer and flux. The high aspect ratio of the CNTs increases the active surface area as well, which may contribute to an increase in flux.
In general, incorporating CNTs in a membrane and/or membrane system is very challenging (e.g., without covering the active surface of the CNTs with the polymer). For example, in polymer coated CNTs, the encapsulating film serves as an additional barrier to mass transfer. Thus, despite efforts to date, a need remains for cost-effective, efficient systems and methods for producing membranes or materials having carbon nanotubes introduced and/or immobilized therein, and improved methods for introducing and/or immobilizing carbon nanotubes in membranes or materials (e.g., polymeric membranes). These and other inefficiencies and opportunities for improvement are addressed and/or overcome by the systems and methods of the present disclosure.